A screw extruder is a machine in which material, usually some form of plastic, is forced under pressure to flow through a contoured orifice in order to shape the material. Screw extruders are generally composed of a housing, which is usually a cylindrical barrel section, surrounding a central motor-driven screw. At a first end of the barrel is a feed housing containing a feed opening through which new material, usually plastic particles, is introduced into the barrel. The screw contains raised portions called flights having a larger radial diameter than the screw's central shaft and which are usually wrapped in a helical manner about the central shaft. The material is then conveyed by these screw flights toward the second end of the barrel through a melting zone, where the material is heated under carefully controlled conditions to melt the material, and then passes through a melt-conveying zone, also called a pumping zone. The melted plastic is finally pressed through a shaped opening or die to form the extrudate.
Besides conveying material toward the die for extrusion, the screw is depended upon to perform mixing of the feed material. Very generally, mixing can be defined as a process to reduce the non-uniformity of a composition. The basic mechanism involved is to induce physical motion in the ingredients. The two types of mixing that are important in screw extruder operation are distribution and dispersion. Distributive mixing is used for the purpose of increasing the randomness of the spatial distribution of the particles without reducing the size of these particles. Dispersive mixing refers to processes that reduce the size of cohesive particles as well as randomizing their positions. In dispersive mixing, solid components, such as agglomerates, or high viscosity droplets are exposed to sufficiently high stresses to cause them to exceed their yield stress, and they are thus broken down into smaller particles. The size and shape of the agglomerates and the nature of the bonds holding the agglomerate together will determine the amount of stress required to break up the agglomerates. The applied stress can either be shear stress or elongational stress and generally, elongational stress is more efficient in achieving dispersion than is shear stress. An example of dispersive mixing is the manufacture of a color concentrate where the breakdown of pigment agglomerates below a certain critical size is crucial. An example of distributive mixing is the manufacture of miscible polymer blends, where the viscosities of the components are reasonably close together. Thus, in dispersive mixing, there will always be distributive mixing, but distributive mixing will not always produce dispersive mixing.
In extrusion processes, the need for good dispersive mixing is often more important than for distributive mixing. This is particularly true in the extrusion of compounds which contain pigments which must be uniformly mixed or small gage extrusion such as spinning of fibers or extrusion of thin films.
In screw extruders, significant mixing occurs only after the polymer has melted. Thus, the mixing zone is thought of as extending from the start of the melting zone to the end of the extrusion die. Within this area there will be considerable non-uniformities in the intensity of the mixing action and the duration of the mixing action, both in the barrel section and in the extrusion die. In molten polymer, the stress is determined by the product of the polymer melt viscosity and rate of deformation. Therefore, in general, dispersive mixing should be done at as low a temperature as possible to increase the viscosity of the fluid, and with it, the stresses in the polymer melt.
Fluid elements are spoken of as having a "mixing history", which refers to the amount of elongational and shear stress to which it has been exposed, and the duration of that exposure. A polymer that melts early in the mixing zone process will have a more significant mixing history than one that melts near the end of the melting zone.
Generally, in an extruder with a simple conveying screw the level of stress or the fraction of the fluid exposed to it is not high enough to achieve good dispersive mixing. Distributive mixing is easier to achieve than dispersive mixing, but unmodified screws have also been found to produce inadequate distributive mixing for many applications. Therefore, numerous variations in screw design have been attempted in prior inventions to increase the amount of distributive or dispersive mixing in screw extruders. These devices usually contain a standard screw section near the material input hopper, and one or more specially designed sections to enhance mixing. These mixing sections naturally fall into the categories of distributive and dispersive mixing elements.
Varieties of distributive mixing elements are shown in FIG. 2 A-F. Practically any disruption of the velocity profiles in the screw channel will cause distributive mixing. Thus even simple devices, such as the placement of pins (see FIG. 2A) between the screw flights can enhance distributive mixing. FIG. 2B shows the well-known Dulmage mixing section, in which the polymer flow is divided into many narrow channels, which are combined and divided again several times. The Saxton mixing section (FIG. 2C) and the "pineapple" mixing section (FIG. 2D) are used to produce similar results. FIG. 2E shows a screw which has slots cut into the flights. A variation called the Cavity Transfer Mixer is shown in FIG. 2F. There are cavities both in the rotor and the barrel. This type of device reportedly performs both dispersive and distributive mixing.
In addition to these devices, static mixers are often used to divide and recombine the melt stream to intermingle the material and eliminate variations in temperature, composition and mixing history. These generally do not provide regions of high stress, and are thus mostly used for distributive mixing.
The devices shown in FIG. 2A-F have been primarily classified as distributive mixers because their action is mainly to spatially redistribute material without subjecting it to regions of high shear stress. The variations shown in FIG. 2G-J are designed to include high shear stress regions and thus perform dispersive mixing.
The most common dispersive mixing section is the fluted or splined mixing section in which one or more barrier flights are placed along the screw so that material has to flow over them. In passing through the barrier clearance, the material is subjected to a high shear rate which acts to break up agglomerates. One such device is the Maddock mixing section, which is shown in FIG. 2G. The Maddock has longitudinal splines that form a set of semicircular grooves. Alternate grooves are open on the upstream and downstream ends. Material that enters the inlet grooves is forced to pass over the mixing flights, which are shown as cross hatched areas, before reaching the outlet grooves. While passing over the mixing flights, the material is subjected to high shear stress. The disadvantage of this type of mixing element is that it reduces the pressure at the output side of the mixing section and thus reduces the output of the extruder. Also there may be regions in which material may stagnate since the grooves have constant depth in a longitudinal direction. This makes it less suitable for materials of limited thermal stability.
FIG. 2H shows the Egan mixing section, which has splines that run in a helical direction to form channels separated by mixing barriers. These channels can have a gradually reducing depth, tapering to zero depth at the end of the mixing section, which reduces the chance of stagnation points. This helical design consumes less pressure than the Maddock style, thus producing less reduction in extruder output.
A blister ring, shown in FIG. 2J, is simply a cylindrical section on the screw that has a small radial clearance, through which all material must pass. This can cause a large pressure drop on the output side of the blister ring, resulting in a significant reduction in overall extruder output.
Screw extruders can have more than one central screw. Twin-screw extruders may operate with two screws that may either rotate in the same direction, or they may be counter-rotating. There are some machines that use more than two screws.
In counter-rotating twin-screw extruders, dispersive mixing primarily occurs in the intermeshing region between the screws. This action is similar to that in a two-roll mill. This configuration has the disadvantage that the mixing action creates substantial separating forces on the screws. These forces can push the screws against the barrel, if these forces grow too great. This can cause wear on the screws and the barrel, thus the screw speed has to be kept low, with resulting decrease in the throughput of the extruder.
In intermeshing co-rotating twin-screw extruders, the screw surfaces in the intermeshing region move in opposite directions. As a result, most of the material bypasses the intermeshing region and moves from one screw to the other repeatedly.
Some twin screw machines have kneading blocks included to increase dispersive mixing. These kneading blocks are most commonly flat paddles of roughly elliptical shape which are stacked on a central shaft, but offset at varying angles. Each paddle on the shaft is paired with a corresponding paddle on the second shaft. The shafts usually both rotate in the same direction but with the angular orientation of the paddles staggered at a certain angle. We can consider the elliptical paddle shapes to have a major and a minor axis with a "tip" on each end of the major axis and a "mid-point" at each end of the minor axis. At one point in the rotation cycle, a tip of a paddle on the first shaft, when horizontally oriented, will nearly contact the midpoint of a paddle on the second shaft, whose tip will then be vertical. As this second, vertical tip rotates towards horizontal, the first tip traces along the elliptical outline of the second paddle, thus "wiping" it. At a further point in the rotation cycle, the second paddle wipes the outline of the first. This wiping action keeps material from stagnating or collecting on the paddle edges. It also imposes constraints on the shapes of the paddles, as the travel of the tip of the neighboring paddle defines the outline of the paddle itself. Although this configuration of paddles can produce fairly good elongational stress in material, the above constraint on the shape of the paddles prevents variations in design, which may produce even better elongational stress regions.
In general, twin-screw extruders are considered to be better at dispersive mixing than single-screw extruders. However for a given capacity, multi-screw machines are usually considerably more costly than single-screw extruders.
For improved mixing to occur, there are several important aspects to be considered. In dispersive mixing, it is the passage of material through a region of high stress that produces the desired breakdown of agglomerates. A single pass through a high stress region will likely achieve only a single rupture of the agglomerate. To achieve a fine scale of dispersion, multiple passes and ruptures may be necessary. Also, for efficient dispersive mixing, stresses in the high stress region should have a strong elongational component, as well as a shear component. For efficient operation of the extruder as a whole, a low pressure drop across the mixing section is desirable. It is also important to combine dispersive and distributive mixing to achieve a more uniform overall mixture. Some distributive mixing occurs whenever dispersive mixing is done, but by deliberately combining distributive elements with the dispersive elements, chances are improved that all fluid elements will pass through the high stress region, preferably many times, for proper dispersion.
To make sure that all agglomerates and droplets pass through high stress regions at least once, the flow rate through the high stress regions must be large enough compared to the overall forward flow rate. This can be done by designing the number of high stress regions, their length and the size of the gap through which material will pass. It is also preferable that there be more than one high stress region, and that these regions are symmetrically arranged around the circumference of any section along the length of the screw, so that forces will be balanced and the possibility of deflection of the screw will be minimized. To reduce pressure drop in the mixing section, it is desirable to have the high stress regions in a forward helical orientation, which can be done by a continuous forward helix or in a stepped forward helix with kneading disks.
For the foregoing reasons, there is a great need for a screw extruder which provides better dispersive mixing than in presently available extruders.